1. Field of the Invention
The present invention relates to a semiconductor device having a nonvolatile semiconductor memory element capable of writing, erasing, and reading, and a method for manufacturing the semiconductor device.
2. Description of the Related Art
A nonvolatile semiconductor element is a semiconductor element capable of electrically rewriting and storing data even when a power supply is turned off. As the nonvolatile semiconductor memory element, nonvolatile memory elements each having a structure similar to that of a metal oxide semiconductor field effect transistor (MOSFET) are classified into two major groups in terms of a difference in a charge trapping film. One includes a floating-gate (FG) type in which a charge trapping film is formed of a conductive film between a channel formation region and a gate electrode; and the other includes a metal-oxide-nitride-oxide-silicon (MONOS) type and a metal-nitride-oxide-silicon (MNOS) type each of which uses an insulating film as a charge trapping film.
In many of the MONOS memory elements and the MNOS memory elements, a silicon nitride film formed by a chemical vapor deposition method is used as a charge trapping film. It is known that a retention characteristic of a memory transistor can be improved by forming a silicon nitride film forming a charge trapping film with a two-layer structure and adjusting a composition or a composition ratio in the two-layer structure (see References 1 to 3).
In Reference 1 (Japanese Examined Patent Application Publication No. S59-24547), a silicon nitride film having a two-layer structure, which is formed by stacking a silicon nitride film which contains comparatively a lot of Si and a silicon nitride film which contains comparatively a lot of N, is used as a charge trapping film. In order to make the upper layer and the lower layer different in concentrations of Si and N, the flow ratio of NH3 to SiH4 at the time of forming a silicon nitride film by a chemical vapor deposition method is changed.
In Reference 2 (Japanese Published Patent Application No. 2002-203917), a charge trapping film formed of a silicon nitride film having a two-layer structure is used. Charge trap density of a nitride film in the upper layer is made higher than that of a nitride film in the lower layer. To form such a charge trapping film, a silicon source gas is varied; thus, two kinds of silicon nitride films are formed, which are different from each other in concentrations of a Si—H bond and a Si—Cl bond.
The following is described in Reference 3 (Japanese Published Patent Application No. 2004-221448): a silicon nitride film with a Si—H bond density of less than or equal to 1×1019 cm−3 is formed by a reduced pressure chemical vapor deposition method using SiCl4 and NH3 as a source material in order to improve a charge retention characteristic of a charge trapping film.
A chemical vapor deposition method (hereinafter, referred to as a “CVD method”) is a method for forming a thin film made of a given substance by decomposing a source gas including a constituent element of a film to be formed, as a composition, and subjecting the source gas to chemical reaction. In order to form a silicon nitride film by a CVD method, SiH4, SiCl4, or the like is used for a silicon source gas, and NH3 is used for a nitrogen source gas. The silicon nitride film which is formed using any of these source gases includes H or Cl which has been included in these source gases. Therefore, the silicon nitride film is formed from silicon nitride with a nonstoichiometric composition, in which a composition ratio of Si to N is out of a stoichiometric composition ratio. Further, as mentioned in References 1 to 3, a composition or a composition ratio of a silicon nitride film affects a charge retention characteristic of the silicon nitride film.
In view of the foregoing problems, the present invention is made by developing a silicon nitride film which is suitable for a charge trapping film. It is an object of the present invention to provide a semiconductor device having a nonvolatile semiconductor memory element capable of improving a charge retention characteristic, and a method for manufacturing the semiconductor device.